Semiconductor laser diodes have numerous advantages. They are small in that the widths of their active regions are typically submicron to a few microns and their heights are usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which are required in order to produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability.
High efficiencies are possible with semiconductor laser diodes with pulsed junction laser diodes having external quantum efficiencies near 50% in some cases. Semiconductor lasers produce radiation at wavelengths from about 20 to about 0.7 microns depending on the choice of semiconductor alloy. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (.about.800 nm) which is near the absorption spectrum of common solid state laser rods and slabs made from Neodymium doped, Yttrium-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermally related problems. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated junction temperatures and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode is decreased by increases in junction temperature.
Furthermore, the emitted wavelength of a laser diode is a function of its junction temperature. Thus, when a set output wavelength is desired, maintaining a constant junction temperature is essential. For example, AlGaAs laser diodes used to pump a Nd:YAG rod or slab should emit radiation at 808 nm since this is the wavelength at which optimum absorption exists in the Nd:YAG. But, for every 3.5.degree. C. to 4.0.degree. C. deviation in the junction temperature of the AlGaAs laser diode, the wavelength shifts 1 nm. Thus, controlling the junction temperature and, thus, properly dissipating the heat is critical.
When solid state laser rods or slabs are pumped by emissions from laser diodes, dissipation of the heat becomes more problematic. Since each individual diode is quite small, it becomes necessary to closely pack a plurality of individual diodes into arrays in order to generate the required amounts of input power to the larger, solid state laser rod or slab. However, when the packing density of the individual laser diodes is increased, the space available for extraction of heat from the individual laser diodes necessarily decreases. This aggravates the problem of heat extraction from the arrays of individual diodes.
One known package which attempts to resolve these thermally-related problems includes the use of a thin, thermally conductive ceramic structure, like beryllium oxide, which is bonded to a thermal reservoir. The ceramic structure includes straight grooves cut therein in which the individual laser diodes are placed. A metallized layer extends from groove to groove to conduct electricity therethrough and supply electrical power to each of the plurality of laser diodes. The laser diodes are soldered in the grooves to the metallized layer.
However, this known package has several problems. For example, laser diodes typically have a slight curvature due to the process by which they are made. Placing a curved laser diode in the straight groove of this known package results in additional stress on the laser diode and an uneven solder bond along the length of the laser diode which can lead to failure. Also, the bottom side of the laser diode in the groove, which is the reflective surface, cannot be cleaned after it is installed which may lead to failures. Additionally, most ceramics, even beryllium oxide, have a lower thermal conductivity than conductive metals such as copper or silver. If beryllium oxide is used, further problems arise since it is a toxic material and cutting grooves produces airborne dust particles. Lastly, because it is extremely difficult to test a laser diode by itself (i.e. not in a package), soldering an untested laser diode into a groove results in an array which may have laser diodes that do not provide optimal performance. Extra cost is then incurred for the removal of a poor performing laser diode.
Therefore, a need exists for a highly conductive laser diode package that is easy to assemble, provides structural support to the laser diode during handling and transportation, allows for cleaning of the reflective surface of the laser diode after assembly, provides for testing of individual laser diodes before their assembly into an array, and is produced from non-toxic, inexpensive materials.